... I suspect it's because your class "com.amazonaws.services.ec2.model.StartInstancesRequest" is "shadowing" the EC2 SDK's class of the same name, due to classloader order.
Now unless you are an employee of Amazon, I don't see any good reason why your class should be in "com.amazonaws.services.ec2.model" package.
Can you change your package name and/or class name to not match any SDK class, rebuild and retry?
[PS: Please enclose all code snippets in code tags in future for better readability. This link shows how. I've done it for you this time.]